Method for biomethanation

ABSTRACT

An improved process for the biomethanation of an organic substrate includes treating the substrate in a first reactor to form organic acid anions, passing an aqueous preparation containing dissolved organic anions through an anion exchanger so that the organic acid anions are adsorbed and separated from the remainder of the aqueous preparation, desorbing the organic acid anions and passing the desorbed acids to a second reactor containing methanogenic bacteria which convert the acids to methane. In a preferred embodiment, a bicarbonate solution is produced in the second reactor and it is used to desorb the organic acid anions and regenerate the anion exchanger into the bicarbonate form.

FIELD OF THE INVENTION

The present invention relates to the microbial biomethanation of organicsubstrates. More particularly, it relates to an improved anaerobicmicrobial biomethanation process for the production of methane fromorganic materials and an apparatus for performing the method.

BACKGROUND OF THE INVENTION

Biomethanation is a process by which organic matter is decomposed by thesimultaneous action of mixed microbial consortia to form methane andcarbon dioxide. The microorganisms in these consortia can be classifiedinto three major trophic groups, namely:

I. hydrolytic fermentative bacteria;

II. syntrophic H₂ -producing acetogenic bacteria;

III. H₂ and acetate consuming methanogenic bacteria.

The hydrolytic fermentative bacteria of group I can convert a complexorganic biomass into simple organic fermentation products includingethanol and organic acids, which are also referred to as volatile fattyacids (VFA's), such as formate, acetate, lactate, propionate, butyrateand benzoate, and hydrogen (H₂) and carbon dioxide (CO₂) gases. Thesyntrophic H₂ producing bacteria of group II further degrade the organicacids and alcohol into the methanogenic precursors acetate and H₂, andthe methanogenic bacteria of group III form methane and carbon dioxidevia acetate-cleavage and consume H₂ by reduction of CO₂ to methane.

These trophic groups constitute a complete microbial food chain and theyperform some unique functions like the interspecies H₂ -transferprocess. The interspecies H₂ -transfer process in abiomethanation-ecosystem is an essential ecophysiological interactionbetween the H₂ -producing acetogenic bacteria (trophic group II) and H₂-consuming methanogenic bacteria (trophic group III), where H₂ is anintermediate produced by the trophic group II and it is consumed bytrophic group III. H₂ even at very low concentrations inhibitsmetabolism of trophic group II. Thus, the simultaneous H₂ -consumingactivity of methanogenic bacteria in the biomethanation ecosystem(trophic group III) must maintain H₂ -partial pressures low enough toallow growth and metabolism of trophic group II by a so called"syntrophic growth".

Biomethanation has been used in waste treatment to get rid of organicwaste.

Prior art anaerobic waste treatment systems (anaerobic contactdigestors) used a non-engineered complex microbial ecosystem for wastebiomethanation that achieves efficient biological oxygen demand (BOD)removal under anaerobic conditions with methane as a byproduct. However,slow growth of the involved microflora and poor methane productivitiesmake it impossible to operate anaerobic waste treatment plants close tothe theoretical biological limits of the ecosystems. Resulting largedigestor volumes with high investment costs limit the economicapplicability of the technology to high volume waste streams.

The so called "advanced reactor concepts"" improve the methaneproductivities by either recycling of the active bacterial biomass intothe process (sludge recycle) and/or the attachment of the activemicrobial flora to support materials (anaerobic filters). A two stagetreatment system where the microbial ecosystem is separated into twopopulations contained in two different reactors connected by a commonliquid phase and operated at acidic pH-values (acidogenic stage) andneutral pH-values (methanogenic stage) has been proposed to improvebiomethanation rates. However, the desired methane (CH₄) productivitiesare rarely obtained. The prior art two stage biomethanation processes donot appear practical because they require high capital costs and twodifferent stages. Upflow-anaerobic-sludge-blanket reactors(UASB-reactors) can render possible high methane productivities becausethe different microbial trophic groups are associated in discretemicrobial granules. However, this system is not used widely because ofprocess instabilities caused by toxic substrate, substrate overloading,nutrient inadequacy and unknown factors which disrupt granulation.

The previously described "advanced reactor concepts" do not involve amicrobiological improvement of ecophysiological interactions during theprocess of biomethanation. The potentially great improvements inbiomethanation processes that can be achieved by eco-engineering thephysiology of the biocatalysts (i.e. creating the optimum environmentfor the mixed biocatalyst) is prohibited by present reactor concepts. Inprior art processes all the trophic groups are either all placed in onereactor (fixed bed or granules) or in two separate reactors linked by acommon liquid phase; the common liquid phase restricts the possibilityfor either selective improvements in a single reaction step or microbialinteractions because the liquid phase is in contact with all thedifferent trophic groups.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to disclose an improvedbiomethanation process which can be used with a broad spectrum of wastesubstrates, and which enhances the specific volumetric reaction ratesfor the biomethanation of organic matter, and removes toxicantsinhibitory to biomethanation.

The biomethanation process of the present invention is a multiple stageor step process which utilizes an apparatus comprised of two independentreactors linked by at least one anion exchanger that functions toconcentrate a carbon source, such as organic acids and alcohols, fromthe first reactor for use in the second reactor where the carbon sourceis converted to methane. The anion exchanger also separates undesiredtoxicants from the carbon source that could interfere with the secondstage of the process.

In the apparatus used in the present inventive process there is nodirect or common liquid interface between the two reactors. The use of afirst independent reactor for the hydrolytic process and a secondindependent reactor for the syntrophic and methanogenic processespermits optimal independent process design for the independent reactorsin all the following respects:

temperature

pressure

pH

support materials

medium composition

microbial composition

essential nutrient requirements

individual dilution rates

Thus, the process of the present invention overcomes the key weaknessesof the prior art processes namely, process instability and low methaneproductivity.

We have discovered that the best point for a stage separation in abiomethanation process involving complex microbial food chains is thecarbon transfer between the trophic groups I and II. Therefore, in theprocess of the present invention, the food chain is separated into anacidogenic stage in the first complex organic hydrolysis reactor (COHR)and a methanogenesis stage in the second syntrophic-biomethanationreactor (SBR). The first and second reactors are connected by the anionexchanger(s); however, there is no common liquid phase connecting thereactors.

The process takes advantage of the common properties and nutritionalneeds of trophic group II and III and the basic imcompatibility of thesetrophic groups with the properties and needs of organisms of trophicgroups I (optimum pH-range, doubling times, anaerobiosis, essentialnutrients). It further provides the essential prerequisite for aninterspecies H₂ transfer by including H₂ -producing acetogenic bacteriaand methanogenic bacteria in the same second reactor (SBR) in form of amicrobial aggregate.

The reaction products of the first stage and the first reactor areeither anions of organic acids (formate, acetate, lactate, propionate,butyrate and benzoate) or alcohols which are converted into such anionsby consecutive reactions. The major soluble endproduct of the concertedaction of the trophic groups II and III in the methanogenic stage isbicarbonate (Table 1). So anion-exchangers can be used to provide asubstrate shuttle probess employing the bicarbonate thus produced todesorb the organic acid and to recharge the anion exchanger.

In the preferred process, the fermentation product from the hydrolysisstage in the first reactor is first filtered to separate the solid phasefrom the liquid phase and the liquid phase containing the dissolved acidanions is passed over an anion-exchange material presented in thebicarbonateform. The anion-exchange material has a greater selectivityfor bicarbonate than for the organic acid anions. The pH-value of thefermentation broth is loW (pH=5-6), therefore, the bicarbonate ions,which dissociate from the anion-exchanger, are protonated and partlydisintegrate into CO₂ and H₂ O. As a consequence of this, the dissolvedorganic acid anions bind to the anion-exchange-material and are removedfrom the rest of the liquid phase. The residue of the liquid phase iseither discarded or recycle to the first reactor.

The separated solid phase includes microbial cells, cell aggregates,organic or inorganic support materials, on the surface of which themicrobial cells are attached and organic particulate carbon substrates.It can be used as an animal feed, biocatalyst or starter culture sourceor recycled to the first reactor.

In the second reactor there is a dense particulate syntrophic andacetoclastic consortium comprising aggregates of microorganismsbelonging to the trophic groups II and III, which are either attached tothemselves or to particles and which preferably have a buoyant densityof greater than 1.05 g/cm³. The consortium can be prepared by treatingsludge from a prior biomethanation process in a continuously operatingupflow reactor with a near neutral growth medium (e.g. PBB) containingessential minerals, a low concentration of inorganic phosphates andsulfides and at least 5 m Mol/l each of at least three of the organicacids to be converted at an increasing feed rate. The microorganisms inthe second syntrophic methanogenic reactor convert the organic acidanions to CH₄, CO₂, and HCO₃ ⁻ at essentially neutral pH-values(pH=6.6-7.8). Thus, the liquid phase in the second reactor contains highconcentrations of dissolved HCO₃ ⁻, typically 50-100 mM.

As previously indicated the selectivity of the anion-exchange materialis higher for bicarbonate than for the organic acid anions and theorganic acid anions are replaced from the anion-exchange material by theaddition of bicarbonate. The aqueous liquid product from the secondreactor is the preferred source of the bicarbonate solution which can bepassed through the anion-exchange material.

In the anion exchanger process, some of the bicarbonate solution also isremoved from the second methanogenesis reactor and introduced into thehydrolysis stage in the first reactor. Thus, the process provides at thesame time base for the hydrolysis stage in the first reactor and acidfor the second reactor because carbonic acid is a weaker acid than theorganic acid biomethanation substrates. This also maintains a stablepH-difference between both reactors.

A very important property of the anion-exchange substrate shuttleprocess is shown in Table 2. Due to the law of mass and chargeconservation, an electroneutral anion substrate shuttle system improvesthe CH₄ -content of the produced biogas because for each mole organicacid converted, one mole of bicarbonate is removed from themethanogenesis stage. Since acetate and propionate are the dominantorganic acids in biomethanation processes, methane contents greater than90% can be realized with the proposed anion-exchange substrate shuttlesystem (Table 2). Thus, the process increases the potential economicvalue of the produced gas by separating the two gases which upgrades theBtu content of the methane gas and produces a separate clean CO₂ gas.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view of the preferred apparatus of the presentinvention for a batch process; and,

FIG. 2 is a schematic view of the preferred apparatus for a continuousprocess.

DESCRIPTION OF PREFERRED EMBODIMENT

The preferred embodiment of the invention will be described inconnection with the drawing in which the solid lines represent tubing.The apparatus of FIG. 1 which is for use in a batch process includes afirst reactor 10 for the complex organic hydrolysis. It may be of anysuitable type and it is operated at a pH of 5.5-6.5 to produce anorganic acid anion rich liquid, which is pumped via pump 11 throughtubing to a filter 12 which has the ability to separate the solid phaseconsisting of suspended material, including cells from the liquid phase.The filter 12 is preferably a membrane filter.

The filtrate containing theorganic acid anions is then passed over anionexchange material in an anion-exchanger 13, which is provided in thebicarbonate form, to adsorb the organic acid anions and leave a liquidof neutral pH, which is essentially free of organic acids and rich inbicarbonate. The thus obtained liquid is then either recycled into thehydrolysis reactor 10 via tubing 14, valves 15 and 16 and tubing 17, 18for pH control or it can be released into the environment as a treatedwaste via tubing 14, 17 and valves 15 and 16.

The filtration step also produces a solid phase or paste rich in cellsfrom the fermentation product of the first organic hydrolysis reactor10; the solid phase can either be recycled into the first reactor 10 viatubing 17 and 18 and valves 15 and 16 to increase the concentration ofactive biomass or removed from the apparatus via tubing 17 and valves 15and 16 to be used as animal feed, a starter culture or a biocatalyst forfurther biotechnological conversions.

The organic fatty acid anions on the anion exchange material in theexchanger 13 can be desorbed with biocarbonate solution and the desorbedacids transported via valve 19 and tubing 20 to the second reactor 21where they are converted by the methanogenic microorganisms to CH₄, CO₂and HCO₃ ⁻. The methane which can be used as fuel leaves the reactor 21via the outlet 26. The process can be repeated by passing another batchof organic acid containing-liquid from the first reactor through theanion exchanger.

The bicarbonate solution used to desorb the organic acid anions may beintroduced into the apparatus via inlet 22 and valve 23 or recycled fromthe second reactor 21 via pump 24, tubing 25 and valve 23.

In FIG. 2 an apparatus for a continuous process is shown. It differsprimarily from the apparatus of FIG. 1 in that there is a second anionexchanger 27 and additional pumps, valves and tubing connecting thesecond exchanger into the system.

In operation the second anion-exchanger 27 of the apparatus of FIG. 2 isinitially loaded with organic acid anions and they are desorbed and theanion exchange material put back in the bicarbonate form by usingfiltered bicarbonate solution from the syntrophic methanogenic secondreactor 21. The desorbed organic acid anions are transported from theanion exchanger 27 via tubing 28 and valve 29 into the second reactor 21where the organic acids are converted to CH₄, CO₂ and HCO₃ ⁻ (see Table1). With proper timing the flow of the first reactor can be passedalternately through two anion exchangers 13 and 27 to provide acontinuous process.

In the continuous process apparatus, the flow of filtrate from thefilter 12 is controlled by the valve 30 which can be operated to directflow to either the first exchanger 13 or the second exchanger 27 viatubing 31. Bicarbonate can be introduced into the exchanger 27 via theinlet 32 or directed to it from reactor 21 via tubing 25, 37 and valves33 and 35. The organic acid free residue of the filtrate leavingexchanger 27 can be returned to the first reactor or removed from thesystem via tubing 36 and valve 16.

The second reactor 21 is preferably operated as an upflow reactor inorder to retain the dense particulate syntrophic microorganisms in thereactor, but any type of reactor that can carry out reactions listed inTable 1 is suitable.

In both the apparatus of FIG. 1 and 2 the methane rich biogas from thesecond reactor 21 is collected via outlet 26.

The present invention possesses many advantages over the prior artprocesses and apparatus. The specially preferred continuous process andapparatus by providing process stage separation with the trophic group Ibiocatalysts, i.e. microorganisms, contained in the first hydrolysisreactor and the trophic groups II and III contained in the secondsyntrophic biomethanation reactor and, by introducing an anion-exchangesubstrate shuttle as the two stage and two reactor coupler the followingadvantages for process operation and stability are achieved:

(a) The novel ability to uncouple the volumetric flows of the hydrolysisand methanogenesis stage and by this to separately improve the dilutionrates of both process stages. Thus, the required reactor sizes andimplicitly the investment costs can be independently minimized for bothwaste treatment stages;

(b) The novel ability to constantly and selectively remove inhibitoryfermentation acids from the first hydrolysis reactor. Thus the specificmicrobial conversion rates of the hydrolysis step improve due to theabsence of end product inhibition;

(c) The novel ability to maintain the organic acid content of aneffluent from a biomethanation process at very low concentrations byanion-exchange treatment and to provide high organic acid anionconcentrations at essentially neutral pH to the second syntrophicbiomethanation reactor. This allows operation under conditions where thetrophic group II and III biocatalysts are fully substrate saturatedwhich dramatically enhances the volumetric biomethanation rates butstill allows the effluent from the overall process to be very low inorganic acids.

(d) The novel ability to protect the very sensitive biocatalysts introphic groups II and III from oxygen, neutral and slightly acidictoxins, antibiotics and other specifically inhibitory compounds, whichare contained in the fluid from the first hydrolysis reactor and, whichdo not bind to the anion-exchange material. Examples for this areammonium or monensin as an antibiotic. This provides higher processstability for the sensitive portion of the biomethanation ecosystem;

(e) The novel ability to design an optimal feed substrate for the secondsyntrophic biomethanation reactor comprised of organic acid anions fromthe first reactor but with low dilution or contamination from the fluidof the hydrolysis stage. Thus, addition of essential growth factors ormetabolic cofactors and minerals for the syntrophic biomethanation ofdilute or concentrated wastes could become economically feasible;

(f) The novel ability to constantly select within a waste treatmentprocess for two separated but dense particulate biocatalysts, oneoptimized for organic hydrolysis to acids the other for the syntrophicbiomethanation of organic acids. This allows the most economic use ofthe expensive reactor volume, as nonparticulate populations with lowervolumetric reaction rates and growth rates would be constantlywashed-out; and, two separate biocatylsts types are optimized forindividual rate limiting steps; and,

(g) The novel ability to produce a biogas with guaranteed methanecontents of greater than 80% without any physical treatment or upgradingof the produced gas. In the case of an anionic-exchange substrateshuttle controlled process, methane contents of greater than 90% areachieved (see Table 2). This opens the way for high grade uses of theproduced methane as a chemical feedstock or a natural gas substitute.The invention is further illustrated by the following examples:

EXAMPLE 1

Anion-exchange resin (55 g of Dowex 1X8) was poured into a glass column(5.5 cm diam.) and was converted to the bicarbonate-form at roomtemperature by passing 3.51 of 1 N NaOH, 3.51 of H₂ O, 200 ml of 1MNaHCO₃ and 500 ml of 0.1M NaHCO₃ over the resin at a rate of 25 ml/min.To simulate the fermentation product of an acidogenic fermentation anaqueous solution containing 50 mM Na-acetate, 50 mM Napropionate and 50mM Na-butyrate at a pH of 5.5 was prepared and passed through the resinat a rate of 3 ml/min. The effluent was collected and analyzed. The pHof the effluent varied between 7.2 and 7.3 and the [HCO₃ ⁻ ] between 50and 80 mM. The organic acid (VFA) contents were analyzed by gaschromatography on Chromosorb 101 (Alltech. Inc.) after acidificationwith 1 N H₃ PO₄. During most of the time only traces of the VFA appearedin the effluent. The exchanged bicarbonate buffered the acidic pH of thesynthetic fermentation liquid, raised the pH of the effluent from 5.5 to7.2 and contained excess bicarbonate (50-80 mM) for further buffering inthe acidogenic hydrolysis process stage.

EXAMPLE 2

An anion-exchange column loaded with acetate, propionate and butyrateidentical to Example 1, was pumped until liquid flow ceased and theneluted with an aqueous solution of 100 mM NaHCO₃ ⁻ (pH=7.6) at a pumprate of 25 ml/min to simulate the fluid composition from a synthrophicbiomethanation reactor. The effluent pH (7.05-7.4) was lower than the pHin the NaHCO₃ solution demonstrating the production of process acidduring the anion exchange substrate shuttle process. Dissolved VFA'swere analyzed as described in Example 1.

The dissolved VFA-anions concentrations reached approximately 75% of thetheoretically possible value (100 mM total at 100 mM NaHCO₃ ⁻). Thisdemonstrates the rapid and quantitative nature of the desorption ofVFA-anions from the resin by bicarbonate ions. The recovery of VFA fromthe anion-exchange resin was greater than 97%.

EXAMPLE 3

An upflow loop-reactor containing a 28 ml bed of a dense, particulatesyntrophic organic anion biomethanation biocatalyst was fed at 35° C.with a mixture of 100 mM Na-acetate, 55.4 mM Na-propionate and 48.2 mMNa-butyrate dissolved in a medium of essential minerals, 0.05% (w/v)yeast extract and 5 mM sodium-phosphate at pH 4.5. The dilution rate was0.1/h and no substrate shuttle unit was used. In steady state operationa pH-value in the effluent of 7.4 and a [HCO₃ ⁻ ] of 90-100 mM wasmeasured. The produced biogas had a methane content of 72%. Themethane-productivity was under standard conditions (1 atm, 0° C.) 80-100VCH₄ /Vbed/d. The characteristic parameters of the VFA-conversion areshown in Table 3. The removal of VFA-anions was greater than 90% showingthat this reactor had all the properties needed for the second reactorof an anion-exchange substrate shuttle process.

The [HCO₃ ⁻ ] (100 mM) and the amount of CO₂ produced during steadystate operation (100 mM) would be sufficient to provide the secondreactor with concentrations up to 100 mM of VFA-anions via a substrateshuttle process (see Example 2).

EXAMPLE 4

An upflow loop-reactor identical to Example 3 with a basically identicalmedium composition was fed at different pH-values in shock loads withconcentrations of all three VFA simultaneously above 10 mM and up to 50mM to saturate the respective second reactor biocatalysts. The pH wasadjusted by varying the dilution rate and adding neutral solutions ofthe sodium salts of the respective missing VFA-anion. Very high methaneproductivities up to 140 VCH₄ /Vbed/d were recorded at each pH value formore than 1 h over a broad pH range between 6.6 and 7.6. This shows thatthe second reactor biocatalysts had considerable stability againstVFA-overloading and pH variations. The very high methane-productivityfurther demonstrates the comparatively high VFA-removal capacity of thesyntrophic organic acids biomethanation reactor.

Representative biocatalysts that can be employed include the followinggenera:

(a) Group I - hydrolytic fermentative bacteria such as Clostridium,Pseudomonas and Enterobacter;

(b) Group II - syntrophic H₂ -producing bacteria such asSyntrophobacter, Syntrophomonas, Clostridium, and Desulphovibrio; and

(c) Group III - methanogenic bacteria such as Methanobacterium,Methanosarcina and Methanothrix.

Those skilled in the art will be aware of the anion exchange materialsthat can be used. In general any anion exchange material that willadsorb the organic acid anions and allow the anions to be desorbed canbe employed. Especially preferred are the anion exchangers that can beplaced in the bicarbonate form to adsorb the anions and which allow theanions to be desorbed with bicarbonate.

                  TABLE 1                                                         ______________________________________                                        Balanced methanogenic conversions in the SBR reactor of                       major selected organic acid anions produced in the COHR                       reactor provided by the anion-exchange substrate shuttle                      component.                                                                                             gas composition                                      reaction                 (% methane)                                          ______________________________________                                        acetate.sup.-  + H.sub.2 O--CH.sub.4 + HCO.sub.3.sup.-                                                 100%                                                 4 propionate.sup.-  + 6 H.sub.2 O--7 CH.sub.4 + 4 HCO.sub.3.sup.-                                      87%                                                  CO.sub.2                                                                      2 butyrate.sup.-  + 4 H.sub.2 O--5 CH.sub.4 + 2 HCO.sub.3.sup.-  +                                     83%                                                  CO.sub.2                                                                      8 lactate.sup.-  + 8 H.sub.2 O--12 CH.sub.4 + 8 HCO.sub.3.sup.-  +                                     75%                                                  4 CO.sub.2                                                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Predicted gas composition in the SBR reactor fed with                         typical organic acid mixtures produced in the COHR reactor.                                gas composition (% methane)                                      products as    with substrate                                                                            without substrate                                  methanogenic substrates                                                                      shuttle unit                                                                              shuttle unit                                       ______________________________________                                        homoacetate    100%        50-60%                                             acetate/butyrate (2:1)                                                                       90%         70-75%                                             acetate/propionate (1:1)                                                                     91%         70-75%                                             acetate/lactate (1:1)                                                                        84%         50-60%                                             homolactate    75%         50-60%                                             ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Organic acid anion removal by a syntrophic organic                            acids biomethanation reactor.                                                                             conversion                                        organic feed-     effluent- rate     COD                                      anion   (mMol/l)  (mMol/l)  (Mol/l)  removal                                  ______________________________________                                        acetate 100.2     9.3       3.5      90%                                      propionate                                                                            55.4      8.5       .72      84%                                      butyrate                                                                              48.2      0.1       .72      99%                                      ______________________________________                                    

It will be readily apparent to those skilled in the art that thebicarbonate solution and the organic acids preparations which areintroduced into the anion exchanger to desorb the adsorbed organic acidanions and to desorb the bicarbonate ions, respectively, must be ofsufficient pH to perform the desired functions with the selected anionexchange material. The bicarbonate solution may be an aqueous solutionof an alkali, alkaline metal or ammonia salt and it will normally beequivalent to about 0.1 M sodium biocarbonate and have a pH of about7.6. The organic acid anion containing liquid from the first reactorwill normally have a pH of about 5 to about 6.5.

It also will be apparent to those skilled in the art that the firststage which produces the organic acids may be a biological or chemicaltreatment and in which the temperatures may range from about 1° C. toabout 500° C. The second methanogenesis stage may be conducted fromabout 1° C. to about 99° C. and preferably at 30° C. to about 50° C. Inaddition, the pressures employed can be varied; however, a pressure ofone atmosphere is preferred.

It further will be apparent that a number of additional changes andmodifications may be made without departing from the spirit and scope ofthe present invention. Therefore, the invention should not be limitedexcept by the claims.

I claim:
 1. A process for the biomethanation of an organic substrate tomethane which comprises: treating the substrate in a first reactor toform a liquid containing dissolved organic acid anions; adsorbing saidanions on an anion exchange material; removing the organic acidanion-free residual liquid; desorbing the organic acid anions from theanion exchanger to obtain an aqueous organic acid anion containingpreparation; and transporting said preparation to a second reactorcontaining methanogenic bacteria which consume the organic acid anionsand produce methane.
 2. The process of claim 1 in which the anionexchange material is in the bicarbonate form and bicarbonate solution isused to desorb the anions.
 3. The process of claim 2 in which aqueousbicarbonate solution is produced in the second reactor and it issubsequently used to regenerate the anion exchange material to thebicarbonate form and to desorb the organic acid anions.
 4. The processof claim 1 in which the substrate is hydrolyzed in the first reactorwith hydrolytic microorganisms.
 5. A biomethanation process comprising afirst microbial hydrolysis stage and a second methanogenesis stage, saidprocess comprising the following steps:(a) hydrolyzing an organicsubstrate to form a solid phase and an organic acid anion-containingliquid phase, and separating the liquid phase; (b) passing the liquidphase into contact with an anion exchange material provided in thebicarbonate form to adsorb the organic acid anions and to desorb thebicarbonate, and separating the resulting bicarbonate solution; (c)regenerating said anion exchange material with a second bicarbonatesolution which is of sufficient strength to effect the desorption anddissolution of said acid anions and the simultaneous adsorption ofbicarbonate on the anion-exchange material; (d) subjecting said desorbedacid anions to a methanogenesis stage to convert the organic acid anionsinto methane, carbon dioxide and bicarbonate by action of denselyaggregated H₂ -producing acetogenic bacteria and, H₂ -consuming andacetate-cleaving methanogenic bacteria; and, (e) utilizing some of thebicarbonate produced in said methanogenesis stage as the secondbicarbonate solution.
 6. A process of claim 5, wherein the bicarbonatesolution of step (b) is used to raise the pH of the hydrolysis stage. 7.A process of claim 5, wherein the solid phase from the separation step(a) is recycled to the hydrolysis stage.
 8. A process of claim 5,wherein the anion-exchange material is converted in its bicarbonate formby treatment with an aqueous solution of a bicarbonate salt of alkali,alkaline earths or ammonia.
 9. A process of claim 5, wherein the organicacid anion concentration in the methanogenic stage exceeds 10 mM.
 10. Aprocess of claim 5, wherein the two stages are conducted in independentreactors which do not have a common liquid phase connecting them so thatsaid hydrolysis and methanogenesis stages can be eco-engineered to theindividual pressure optima, the individual temperature optima, theindividual pH-optima, individual optimum loading rates, individualaqueous medium composition and individual optimum nonaqueous mediumcomposition or support materials of each of said process stages withoutinterference from the other stage.
 11. A process of claim 5, wherein apressure of essentially 1 atm is employed and the methane composition ofthe untreated produced gas from the methanogenesis stage exceeds 80%methane due to the bicarbonate removal during regeneration of the anionexchange material.